Transmission-line control system



Feb, 4, 1928.,

R. K. POTTER TRANSMISSIONL LINE CONTROL SYSTEM original Filed Dec. 2, 1925 lllllllli Patented Feb. 14, 1928.

UNITED STATES PATENT OFFICE.

RALPH K. POTTER, OF NEW YORK, N. `Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

TRANSMISSION-LINE CONTROL SYSTEM.

Application :tiled December 2, 1925, Serial No. 72,842. Renewed November 13, 1926.

This invention relates to transmission level control systems and particularly to a system of that type in 'which the ratio of the transmission level of the communication signal to the noise level existing between two stations of the system is maintained constant by a control signal transmitted from the station that is receiving the transmitted signal.

1n transmission level control systems which have for their object the maintenance of a constant ratio between the communication signal level and the noise level over the transmission path between the communicating stations of the system, it is customary to transmit from the communicating stations a control signal, in order to vary the level of the transmitted communication signal in accordance withI variations in the noise level existing at any instant. While both stations are adapted to transmit communication signals and control signals, and also to receive such signals from the other station, only the control signal of the station that is receiving at any instant is employed to control thetransmission level of the communicatlon sig--` nal at the stationl that is transmitting at that instant. Therefore, the transmission of the control signal energy from the station that is transmitting at that instant serves no useful purpose, and is in fact detrimental in that it tends to overload the power amplifier in the transmitting branch ofV the station transmitting at that time.

One of the objects of this invention is to provide means to prevent the generation and radiation of the control signal frequency at each station during the time in which that station is transmitting communication signals to another station or stations of the system. i

Other objects of this invention will be apparent from the following .description when read in connection with the attached drawing, of which Figure 1 shows schematically one form of embodiment of the invention, which figure will be designated station A of the system, and Fig. 2, which is a duplication of Fig. 1, represents a second station designated B of the SYStem.

In Fig. 1, the line L1 represents a wire circuit to which a subscribers telephone may be connected either directly or through the intermediation of c'entral oliices or other switching devices. The line L1 is connected with the transmitting and receiving branches of the terminal station A of the radio system by means of the hybrid coil 1. Associated with the hybrid coil is the artificial line 2 designed to simulate electrically the line L1. This coil is of the well-known type having a plurality of windings so arranged and proportioned that when energy is impressed upon the coil by the line L1, it will pass into the transmitter branch circuit 3, and into the receiver circuit 4:. It will, however, produce no effect in the lattercircuit, since a unilateral device such as a vacuum tube amplifier is connected in that circuit. The transmitter circuit 3 is coupled to a modulator M1 which preferably is of the type in which the carrier frequency is suppressed. This carrier is supplied to the input circuit of the modulator through the transformer 5. The manner in which the carrier frequency is generated and supplied will be later described in detail. The output circuit of the modulator M,l is coupled lW the transformer 6 with the filter 7, which is of a ty`pe designed to suppress one of the bands resulting from modulation. This filter is "connected with an amplifier 8 which in turn is connected by the transformer 9 with the input circuit of the power amplifier 10. Also connected with the input side of this amplifier by the transformer 11 is the circuit 12 which is connected with a source .of control frequency oscillations. The output `side of the amplifier 10 is connected with the antenna T1.

Bridged across the circuit 3 is a winding of the transformer 13, the other winding of which is connected with the input side of an vamplifier 14 .having its output side connected with the winding of a voice-operated relay 15. The function of this rela is primarily to lprevent the generation o oscillations of the control signal frequency during the time in which a communication signal is being transmitted from the antenna T,L of station A, inasmuch as such transmission of the control frequency at that time performs no useful function and simply tends to overload the power amplifier 10 during the time in which the communication signalsoy Ilm

ing wave is passing through this amplifier. The armature of the relay which is normally biased by a spring or otherwise, to remain upon its back contact when the relay is not energized, is connected by conductor 16 with the input side of the control signal oscillator 17, the connection including a source of current 18. The front contact of relay 15 is also connected by conductor 19 with` the input circuit of the oscillator 17, the said conductors 16 and 19 being bridged across the resistance 20 which is shunted by the condenser 21. A source of negative biasing potential 22 is connected with the input side of the oscillator, and a source of plate potential 23, with the output side of the said oscillator. Connected between the plate and the grid is a piezo crystal 24 which functions in a well-known manner to transfer from the plate to the grid only a single frequency which is the frequencyv of the control signal oscillations. It is desirable to point out at this point that when the armature of relay 15 is in the position shown in the figure, a negative potential will be applied by the source 22 to the grid of the oscillator 17, which causes the oscillator to oscillate at the frequency transferable through the crystal 24, and the said oscillator will continue to oscillate at that frequency as long as the armature remains in the position shown. These oscillations will be transmitted by the circuit 12 to the input circuit of the power amplifier 10 through the transformer 11, and will b radiated from the antenna T1. The back contact of the relay 15 closes a circuit between the rid and the filament of the amplifier 25, W ich path shunts the winding of the transformer 26 which connects the filter 27 of the receiving circuit with the said amplifier, the purpose of doing 'which will be clear from subsequent description of the receiving circuit. The output side of the amplifier is connected with the vacuum tube regulator 28, to the input side of which is also connected, through the transformer 29, a source of carrier oscillations 30. These oscillations pass through the filter 31 and thence over the branch circuit 32 to the input side of the modulator M1 through the transformer 5. The receiving circuit of the station comprises a receiving' antenna R, which is connected with a modulator 33 which may be of any type, but preferably is of a type having a source of local oscillations 34 connected therewith to beat down the received frequency to an intermediate frequency, which intermediate fre uency is amplified by the am lifier 35 and lmpressed upon the lter 27. is filter is of a ty e adapted to transmit therethrough two fiequencies or groups of frequencies, impressing them upon their appropriate circuits, depending upon the frequencies involved. Thus the commup nication frequency or band passes to the circuit 36 and is impressed upon the detector 37 which reduces the intermediate frequency to an audio frequency and transmits the communication signal to the amplifier 38 and thence to the line Ll-through the hybrid coil 1. The other frequency which comes into this receiving circuit, namely' the control signal frequency, is diverted by the filter 27 into the branch circuit 39-which is coupled by the transformer 26 with the input side of the amplifier 25. i

Another station of this system, which has been designated B, is shown in Fig. 2. In this figure, which is a duplicate of the circuit shown in Fig. 1, the same reference figures have been used with the addition of prime marks to distinguish between the figures.

The manner in which the invention attains its object will probably be better understood by considering first the manner in which a. communication signal, such as a speech signal, is transmitted from station A to station B, and then by considering the manner in which the control signal is transmitted and received at the respective stations.

lVhen a band of speech frequencies is impressed upon the line L1 and transmitted thereover to station A, these fre uencies will be impressed by the hybrid coi 1 into the transmitting circuit 3, and voltages will be set up across the transformers 13 and 40. The voltage set up across the transformer 40 will serve to modulate the high frequency oscillations which are impressed upon the common conductor of l the modulator M1. These high frequency oscillations which are produced by the device 30 are in'turn impressed by the transformer 29 upon the input circuit ofthe regulator 28 which is in the nature of a power limiting device, and they pass through the filter 31 and over the branch circuit 32 to the input side of the modulator. The high frequency speech signal will 4then be impressed by the transformer 6 upon the filter 7, which suppresses one side band land impresses the other upon the amplifier 8, which in turn impresses the band upon the power amplifier 10. The speech signal then passes to the antenna T1 and is radiated to the distant station B. At the distant station it is recelved by the antenna R1 and is beaten down to an intermediate frequency by the modulator 33 and amplified by the amplifier 35. This intermediate frequency band carrying the speech signal passes` through the filter 27 to the circuit 36', and the speech band is detected by the detector 37 This speech band is amplified by the amplifier 38 and is impressed upon the line L',L through the hybrid coil 1.

A small part of the speech energy in the circuit 3 at station A passes throu h the transformer 13 to the amplifier 14, w1th the plate circuit of which the winding of the neueste voice-operated relay'is connected. This rclay will, accordingly, be operated by attracting its armature to its front contact. The effect' of this is to close a circuit from the source of current 18, over conductor 16, through the front contact of relay 15, thence over conductor 19 and through the resistance 2O t-o the said source of potential. vIt

will, accordingly, be seen that this current vtransmitting to station B, the communication signal lthat is received over the line L, at station A, there will be no radiation from the antenna T1 of a control signal, inasmuch as the voice frequency impressed by the line L1 upon the transmitting circuit 3 energizes the voice-operated relay 15 and closes the circuit which in part contains the source of potential 18 and the resistance20, and stops the product-ion of oscillations by the source 17.

Since station B is at this instant receiving the speech signal from station A, the voiceoperated relay at station B will not be energized, and consequentlythe source of the control signal oscillations 17 at station B Will be operating, impressing its oscillations upon the circuit 12 and thence upon the input circuit of the power amplifier 10 through the transformer 11. This control signal is radiated by the antenna T1 and is received by the antenna R1 at station A during the time in which the speech signal is being radiated from station A, both of the signals traversing substantially the same path between the two stations, but. in opposite directions. The control signal received by the antenna R, is beaten to an intermediate frequency by the device 33, which intermediate frequency signal is amplified by 35, and by the action of the filter 27, this control signal is diverted into the branch circuit 39 and impresses a voltage depending upon the magturn applied to the grid of the regulator 28, to the input side of which is connected the source of carrier oscillations. In this manner, the amplitude of the carrier oscillations is 'varied by the received control signal oscillations. If the said control signals are greatly attenuated in transmission'from station B to station A, the amplitude of the car rier oscillations produced by 30 will be increased by the regulator 28, and consequently the level of the speech signal produced by the modulation of the carrier oscillations by the incoming speech band will be raised in order to maintain it well above the level of Ainterference between station A and statinv B. As soon as the subscriber connected with the line L1 ceases talking and begins to listen to the subscriber connected with the line L1 at station B, the voice-operated relay 15 at A will be deenergized, thereby putting a shunt across the Winding of the transformer 26 which renders thisdevice ineffective' and thereby reduces the intensity ot the carrier oscillations applied to the modulator M1 during the interval in which speech signals are not being transmitted from the antenna T1. Furthermore, since the relay v15 is deenergized, the control signal oscillator 17 begins to function and transmit from the antenna T1 to the receiving circuit at station A, a control signal to effect the proper regulation of the speech signal being transmitted from Astation B to station A at that instant. This speech signal `passes through the receiving circuit from the antenna Rl to the hybrid coil 1,.and is there impressed upon the line L1 for transmission to the listening subscriber.

While this invention has been disclosed as embodied in a particular form, it is obvi'ously capable of embodiment in other forms Without departing from the spirit and scope of the appended claims.

lVhat is claimed is:

1. In a signaling system having a signal input circuit, a high frequency modulating and transmitting circuit, and a 'source of control signaling oscillations, the method for preventing the transmission of control signal oscillations during the transmission of a. communication signal which consists in receiving a signal current in the signal input circuit, diverting a-portion of the said signaling current, modulating a carrier frequency by the remaining portion of the signal current, and effectively stopping the production of contrql signal oscillations by the diverted portion of the signal current.

2. In a signaling system having a signal input circuit, a high frequency modulating and transmitting circuit, and a source of control signal oscillations, the method for preventing the transmission of control signal oscillations during the transmission of a communication signal which consists in relUU ceiving a signal current in the signal input circuit, diverting a portion of the said signaling current, modulating a carrier frequency by the remaining portion of the signal current, and effectively stopping the transmission of the said control signal oscillations by the diverted portion of the signal current.

In a signaling system having a signal input circuit, a high frequency modulating and transmitting circuit, and a source 0I control signal oscillations, the method for preventing the transmission of control signal oscillations during the transmission -of a comiminication signal which consists in receiving a signal current in the signal input circuit, diverting a portion of the said signaling current, modulating a carrier frquency by the remaining portion of the =signal current, amplifying the said diverted portion to such degree as is necessary to operate an electro-responsive device, and controlling the production of control signa] oscillations by the operation of the said device.

4. In a. signaling system comprising a plurality of stations cach having a transmitting branch, a receiving branch, a source of control signal oscillations, a source of carrier oscillations and a control circuit therefor,the method of controlling the level of transmission between two stations Which consists in receiving at one station a signal current, modulating the carrier oscillations by the said|signal current to produce a communication signal, and transmitting the said communication signal to a. second station, simultaneously diverting a portion of thesignal current from its source and effecting by the said diverted current the prevention of transmission of control signal oscillations from the said first station, transmitting from thesecond station control signal oscillations, receiving the said control signal oscillations at the said first station, and effecting the control of the amplitude of the said carrier oscillations by the received control oscillations.

5. In a signaling system characterizedby a source of control signaling oscillations at each of the stations for controlling the transmission level of communication signals transmitted between stations of the said system, the method for preventing the transmission of a. control signal from one station of the system to a second station during the interval in Which the said first station is transmitting a communication signal which consists inreceiving at the first station a signal, modulating the carrier Wave bythe said signal and transmitting the Wave as thus modulated, diverting a portion of the signal energy from the said source ofthe signal, and controlling the generation of the control signal frequency at the said first station by the said diverted energy. t

6. In a signaling system, the combination with a high frequency transmitter having a modulating device, of a source of carrier oscillations connected therewith, a source of signals to modulate the said carrier oscillations also connected with the said modulating device, a source of control signal oscillations, and means connected with the said source of signals to control the operation of the `said source of control signal oscillations.

7 The methd of maintaining substantially constant the ratio of the signal level to the noise level in a signaling system, which consists in transmitting a signal Wave from A to B, simultaneouslytransmitting a control yWave from B to A, the frequency of the con.-

trol wave differing from that of the signal Wave, and varying the amplitude of ,the signal wave as transmitted from A-by and in accordance With changes in the amplitude of the control Wave as received at A to maintain constant the ratio of the signal level to the noise level throughout the system.

8. The method of maintaining substantially constant the ratio of the signal level to the noise level in a signaling system, which consists in transmitting a signal Wave from A to B, simultaneously transmitting a control Wave from B to A, traversing substantially the same path as the signal Wave and eiiposed to the same energy-dissipatmg forges, the frequency of the control Wave differing from that of the signal wave, receiving the control Wave at A and varying the level of the transmitted signal Wave inversely with changes in the level of the control Wave as received.

9. In a'signalingsystem I(':haracterized by a plurality of stations connected by a t -ansmittingmedium in Which disturbing oltages may be created by sources foreign to the system itself, the method of maintaining substantially constant the ratio of the signal level to the disturbing voltage level, which consists in transmitting a signal Wave from one of the stations of the said system to another station thereof, simultaneously transmitting a control Wave from the said other station to the said first station, the said control wave differing in frequency from the said signal Wave, and varying at the said first station the level of the signal Wave as transmitted inversely with the level of the c ontrol Wave as received at the said first stat1on.

10. The method for maintaining substantially constant the ratio of the signal level to the noise ,level in a signaling system embracfng a plurality of stations, which consists in generating a carrier wave at one of the said stations, modulating the said carrier by a signal, transmitting a side band of such ting to the said first mentioned station a control wave having constant aniplitudeas transmitted, and d'rected along substantially the same path as the said side band transmitted fiom the first mentioned station, receiving he said control Wave at the first mentioned station and eifectively governing the amplitude of the side band by the said control wave as thus received.

11. The method for maintaining substantially constant the ratio of the signal level to the noise level in a signaling system embracing a plurality of station-s, which Consists in generating a carrier Wave at one ot' the said stations, modulating the said carrier by a signal, transmitting aside band of such modulation and suppressing the transmission of the unmodulated carrier Wave, transmitting to the said first mentioned station a control Wave having constant amplitude as transmitted, and directed alongI substantially the same path as the said side band transmitted from they first mentioned station, receiving the said control Wave at the irst mentioned station and varying the amplitude of the carrier employed in modulation at the irst mentioned station inversely with changes in the amplitude of the control wave as received thereat.

12. The method for maintaining substantially constant the ratio of the signal .level to the noise level in a signaling system embracing a plurality of stations, which consists in generatinga carrier Wave and a control Wave at each of the said stations between which communication is to be maintained, modulating the said carrier at one of the communicating stations, transmitting a side band resulting from the said modulation and preventing the transmission of. the iinni'odulated carrier, simultaneously transmitting from the other of the communicating stations the conltrol Wave, receiving the said control Wave at-the first mentioned station, and controlling the amplitude oi the side band by the control Wave as received at the first mentioned station.

13. The method for maintaining substantially constant the ratio of the signal level to the noise level in a signaling system embracing a plurality of stations, which con sists in generatinga carrier Wave and a control Wave at each ot the said stations between *which communication is to be mairitained, modulating the said carrier at one oit the communicating stations, transmitting a side band resulting from the said modulation and preventing the transmission oit nnrnodulatcd carrier, simultaneously traviy itting from the other ot the commuf stations the control vvave, receiving` d. contrai at the mentioned station, and varying the amplitudeof the Carrier employed in modulation at the first mentioned station inversely with the changes in amplitude of the control Wave as received theieat.

14. The method for maintaining` substantially constant the ratio ot the signal level to the noise level in a signaling system embracing a plurality of stations, which consists in the simultaneous transmission of signaloscillaticns and control oscillations in opposite direction between stations of a system, the control oscillations differing in frequency from the signal oscillations, receiving the control oscillations at the station transmitting the signal oscillations, separating the control oscillations from other oscillations that may be present at the last mentioned station by virtue of the frequency difference existing between the said types of oscillations, and varying the strength of the signal oscillations as transmitted inversely with changes in the strengthV of the control oscillations as received at the station transmitting the signal oscillations.

15,. In a signaling system the combination Witlia transmitting branch having a source of signals, a source of carrier oscillations,

a carrier suppression modulator connected with both of said sources for the production of side bands of the said carrier by the said signal, a transmission medium upon which is impressed one of the said side bands, and a source of transmission level control oscillations, differing in frequency from the carrier oscillations, connected with the said transmission circuit, and a receiving branch having means to separate the signal side band from the. control oscillations, and means responsive, to the amplitude of the control oscillations to govern the amplitude of the carrier oscillations created by the source connected with .the said transmitting branch.

16. In a signaling system comprising a plurality of stations, each having a transmitting branch and a receiving branch, the transmitting branch comprising a source of signals, a source of carrier oscillations. a carrier suppression modulator connected with both of said sources for the production of side bands of the said carrier by the said signal, a sourceI of control oscillations diftering in frequency from the carrier oscillations, a transmission. medium upon which are impressed a signal side band'froni the said modulator and control oscillations, and a receiving` branch comprising inea-ns to separate by ltering the side band and the control oscillations received roni another of the said stations, means to detect the signal embraced by the said side band, anc. means responsive to the `anfiplitiide et the control oscillations to govern the amplitude of the carrier cscillations created bv the' source @asserted rita branch.

17. In a signaling system in which signals are transmitted between stations by carrier oscillations modulated by the said signals, the method of cont-rolling the transmission level of the signals which consists in transmitting control oscillations from a station that is 'then receiving signal modulated carrier oscillations from another station, the control oscillations differing in frequency 10 from ,the carrier oscillations, receiving the said control oscillations at the said other station and varying the amplitude of the carrier oscillations at the said other station by changes in amplitude of the control oscillations received thereat.

In testimony whereof, I have signed my name to this specification this 1st day of December, 1925.

RALPH K. POTTER. 

